Abstract

The pre-existing anti-poxvirus immunity in cancer patients presents a hurdle to successful poxvirus-mediated oncolytic virotherapy. To overcome this hurdle, we have explored strategies of immunosuppression (IS) or/and immune evasion (IE) for successful delivery of an oncolytic vaccinia virus to tumor models in mice pre-vaccinated with a poxvirus. These include transient IS using a cocktail of three immunosuppressive drugs (IS drugs) (tacrolimus, mycophenolate mofetil and methylprednisolone sodium succinate) that have been used in organ transplant, carrier cells as a means of IE, or the combined strategy. The three drug cocktail, which targeted multiple components of innate and adaptive immunity, displayed no enhancing effects on viral infection in tumor tissue in the pre-immunized host. Using B cell knockout mice, we confirmed that B cells and the neutralizing antibodies, the later of which were not targeted by the drug cocktail, played a significant role in preventing poxvirus infection. Cancer cells as carrier cells alone were not effective to deliver the virus to tumor. Instead, the application of both IS drugs and carrier cells worked synergistically to promote the delivery to, viral infection, replication and spread inside the tumor mass and thus enhanced the efficacy of oncolytic virotherapy in an ovarian carcinomatosis model. The administraton of these IS drugs led to facilitated development of tumor-associated macrophages (TAMs) into immunosuppressive M2 phenotype (IL-10hi/IL-12low) in the tumor microenvironment. These results revealed a novel mechanism for induction of the immunosuppressive tumor microenvironment by the IS drugs, and demonstrated in principle that it is feasible to treat pre-vaccinated cancer patients with an oncolytic poxvirus when combined with both IS and IE strategies.